This invention relates generally to medical systems and procedures and more particularly to systems and procedures for effecting revascularization of the myocardium of a living being.
Atherosclerosis is the leading causes of death in the industrial world today. During the disease process, atherosclerotic plaques develop at various locations within the arterial system of those affected. These plaques restrict the flow of blood through the affected vessels. Of particular concern is when these plaques develop within the blood vessels that feed the muscles of the heart. In healthy hearts, cardiac blood perfusion results from the two coronary arterial vessels, the left and right coronary arteries which perfuse the myocardium from the epicardial surface inward towards the endocardium. The blood flows through the capillary system into the coronary veins and into the right atrium via the coronary sinus. When atherosclerosis occurs within the arteries of the heart it leads to myocardial infarctions, or heart attacks, and ischemia due to reduced blood flow to the heart muscle.
Over the past few years numerous methods for treating cardiovascular disease have become available. Traditional methods utilize open surgical procedures to access the heart and bypass blockages in the coronary blood vessels. In these procedures, the patient's heart is surgically exposed and one or more coronary arteries are replaced/bypassed with synthetic or natural bypass grafts. During conventional cardiac surgery, the heart is stopped using cardioplegia solutions and the patient is put on cardiopulmonary bypass which uses a heart-lung machine to maintain circulation throughout the body during the surgical procedure. A state of hypothermia is induced in the heart tissue during the bypass procedure to preserve the tissue from necrosis. Once the procedure is complete, the heart is resuscitated and the patient is removed from bypass. There are great risks associated with these surgical procedures such as significant pain, extended rehabilitation times, and high risk of mortality for the patient. The procedure is time-consuming and costly to perform. This surgery also requires that the patient have both adequate lung and kidney function in order to tolerate the circulatory bypass associated with the procedure and a number of patients which are medically unstable are thus not a candidate for bypass surgery. As a result, over the past few years minimally invasive techniques for performing bypass surgery have been developed and in some instances the need for cardiopulmonary bypass and extended recovery times are avoided. In addition, as an alternative to surgical methods, non-surgical procedures, such as percutaneous transluminal coronary angioplasty, rotational atherectomy, and stenting have been successfully used to treat this disease in a less invasive non-surgical fashion.
In balloon angioplasty a long, thin catheter containing a tiny inflatable balloon at its distal end is threaded through the cardiovascular system until the balloon is located at the location of the narrowed blood vessel. The balloon is then inflated to compress the obstructing plaque against the arterial wall, thereby restoring or improving the flow of blood to the local and distal tissues. Rotational atherectomy utilizes a similarly long and thin catheter, but with a rotational cutting tip at its distal end for cutting through the occluding material. Stenting utilizes a balloon tipped catheter to expand a small coil-spring-like scaffold at the site of the blockage to hold the blood vessel open. While many patients are successfully relieved of their symptoms and pain, in a significant number of patients, the blood vessels eventually reocclude within a relatively short period of time. In addition, for a large number of patients that are in the later stages of ischemic heart disease, the current technology offers little hope for long term cure. In these patients even extending the patient's life for several months provides a significant benefit to the patients and their families.
Although these non-surgical procedures are much less costly and less traumatic to the patient than coronary bypass surgery there are a number of patients for which these procedures are not suitable. For certain types of patients the presence of extremely diffuse stenotic lesions and total occlusion in tortuous vessels prohibits them from being candidates. In addition to these procedures which attempt to reopen or bypass the coronary vessels, direct myocardial revascularization has been performed by inducing the creation of new channels, other than the coronary arteries themselves, to supply oxygenated blood and remove waste products from the heart tissue. Myocardial revascularization is a technique used to supplement the blood supply delivered to the heart by providing the ischemic inner surface of the heart, known as the endocardium, with direct access to the blood within the ventricular chamber. Typically the endocardium receives its nutrient blood supply entirely from the coronary arteries that branch through the heart wall from the outer surface known as the epicardium.
In an article entitled "New Concepts In Revascularization Of Myocardium" by Mirhoseini et al. in Ann. Thor. Surg., 45:415-420, April, 1988 the work of investigators exploring several different approaches for direct revascularization of ischemic myocardium is discussed. One revascularization technique utilizes "myoepexy", which consists of roughening of the myocardial surface to enhance capillarization. Another technique, known as "omentopexy", consists of sewing the omentum over the heart to provide a new blood supply. Another approach involves implanting the left internal mammary artery directly into heart muscle so that blood flowing through the side branches of the artery will perfuse the muscle.
Similar revascularization techniques have involved the use of polyethylene tubes, endocardial incisions, and the creation of perforated or bored channels with various types of needles, and needle acupuncture. For example, T-shaped tubes have been implanted in the muscle, with the leg of the T-tube extending into the ventricular cavity as reported by Massimo et al. in an article entitled "Myocardial Revascularization By A New Method Of Carrying Blood Directly From The Left Ventricular Cavity Into The Coronary Circulation" appearing in J. Thorac. Surg., 34:257-264, August, 1957. In an article entitled "Experimental Method For Producing A Collageral Circulation To The Heart Directly From The Left Ventricle" by Goldman et al. in the Journal of Thoracic and Cardiovascular Surgery, 31:364-374, March, 1965, several experimental methods for myocardial revascularization are described. One method involved the implantation of excised perforated carotid arteries into the left ventricular wall. Goldman et al. also examined the use of implanted perforated polyethylene tubing in a similar fashion.
Needle acupuncture approaches to direct myocardial revascularization have been made and were based upon the premise that the heart of reptiles achieve myocardial perfusion via small channels between the left ventricle and the coronary arterial tree as described by Sen et al. in their article entitled "Transmyocardial Acupuncture: A New Approach To Myocardial Revascularization" in the Journal of Thoracic and Cardiovascular Surgery, 50:181-187, August, 1965. In that article it was reported that researchers attempted to duplicate the reptilian anatomy to provide for better perfusion in human myocardium by perforating portions of the ventricular myocardium with 1.2 mm diameter needles in 20 locations per square centimeter. It has been shown that the perfusion channels formed by mechanical methods such as acupuncture generally close within two or three months due to fibrosis and scaring. As a result these types of mechanical approaches have been abandoned in favor of the use of lasers to effect the transmyocardial revascularization (TMR).
U.S. Pat. No. 5,591,159 (Taheri) describes a device for effecting myocardial perfusion that utilizes slit needles to perforate the myocardium. The needles may also utilize a laser beam directed through the lumens of the needles. The device uses a trans-femoral approach to position the device into the left ventricle of the patient. A plunger is activated to cause the needles to enter the myocardium several times. Perforation of the myocardium may be effected by means of a laser beam through the lumen of the needle or high velocity drill.
U.S. Pat. No. 5,655,548 (Nelson et al.) describes a method for perfusing the myocardium using a conduit disposed between the left ventricle and the coronary sinus. In one method, an opening is formed between the left ventricle and the coronary sinus, and the coronary ostium is partially occluded using a stent that prevents the pressure in the coronary sinus from exceeding a predetermined value. Blood ejected from the left ventricle enters the coronary sinus during cardiac systole. The apparatus limits the peak pressure in the coronary sinus to minimize edema of the venous system. The system utilizes retroperfusion via the coronary sinus of the venous system.
Previous researchers had explored long term retroperfusion via the coronary sinus but found that its leads to edema of the cardiac veins which are incapable of sustaining long-term pressures above about 60 mm Hg. The procedure basically places a stent-like plug in the left ventricle so that blood flows into the coronary sinus and then into the myocardium via the venous system using retroperfusion, not into the myocardium directly. In the aforementioned Nelson et al. patent there is disclosed the use of a cutting instrument, such as a cannulated needle, a rotating blade, or medical laser to provide the required opening for the conduit. It is believed that when implanted in the heart, the plug and stent will result in long-term retrograde perfusion of the myocardium using the cardiac venous system and will cause a redistribution of the flow within the venous system so that a greater fraction of the deoxygenated blood will exit through the lymphatic stem and the Thebesian veins. The inventors also describe the use of a conduit which takes the place of the coronary sinus.
U.S. Pat. No. 4,658,817 (Hardy) describes a surgical carbon dioxide laser with a hollow needle mounted on the forward end of the handpiece. The needle is used to perforate a portion of the tissue, for instance the epicardium, to provide the laser beam direct access to distal tissue of the endocardium for lasering and vaporization. The device does not vaporize the tissue of the outer wall instead it separates the tissue which recoils to its native position after the needle's removal. This technique eliminates surface bleeding and the need for suturing the epicardium as is done with other techniques.
In U.S. Pat. No. 5,607,421 (Jeevanandam) discloses that laser channels remain open because carbonization associated with the laser energy inhibits lymphocyte, macrophage, and fibroblast migration. Thus, in contrast to channels created by needle acupuncture, laser channels heal more slowly and with less scar formation which allows endothelialization and long term patency.
It has been reported by Moosdorf et al. in their article entitled "Transmyocardial Laser Revascularization--Morphologic Pathophysiologic And Historical Principles Of Indirect Revascularization Of The Heart Muscle" in Z Kardiol, 86(3): 147-164, March, 1997 that the transmyocardial laser revascularization results in a relevant reduction of clinical symptoms such as angina and an increase of exercise capacity in approximately two thirds of the patients treated. Objective data of enhance myocardial perfusion as assessed by positron emission tomography, thallium scans, and stress echocardiography has also been presented in other studies. Some researchers have found that TMR channels created by CO2 lasers are surrounded by a zone of necrosis with an extent of about 500 microns. In heart patients who died in the early postoperative period (1 to 7 days) almost all channels were closed by fibrin clots, erythrocytes, and macrophages. At 150 days post procedure, they observed a string of cicatricial tissue admixed with a polymorphous blood-filled capillary network and small veins, which very rarely had continuous links to the left ventricular cavity. At the 2 week post procedure point a granular tissue with high macrophage and monocyte activity was observable. See for example, the article by Krabatsch et al. entitled "Histological Findings After Transmyocardial Laser Revascularization" appearing in J. Card. Surg. 11:326-331, 1996, and the article by Gassler et al. entitled "Transmyocardial Laesr Revascularization. Historical Features In Human Nonresponder Myocardium" appearing in Circulation, 95(2): 371-375, Jan. 21, 1997.
In summary, there a number of potential mechanisms which individually or in combination may be responsible for the improvements seen in patients subjected to the previously described myocardial revascularization techniques including: (1) new blood flow through created channels, (2) angiogenesis (stimulation of the creation of new blood vessels), (3) cardiac denervation, (4) the placebo effect, and (5) ablation of ischemic myocardium.
Currently it is believed that cardiac denervation and angiogenesis are the primary causes for post procedure angina relief and improved perfusion respectively. The injury stimulates vasculargenesis and the laser energy damages nerves thereby minimizing the pain sensation. The lasers are however very expensive to purchase.
While the aforementioned techniques and methods for revascularizing the myocardium offer some promise they never the less suffer from one disadvantage or another.